The present invention relates to a cathode ray tube, and particular to a cathode ray tube capable of shortening its overall length without degrading its focus characteristics by optimizing its electron gun in a single-electron-gun cathode ray tube such as a cathode ray tube used for a projection TV.
Generally, a projection TV receiver uses three cathode ray tubes exclusively for reproduction of red, green and blue primary color images, respectively, magnifies and projects three primary color images (a 5.5-inch diagonal phosphor screen, for example) provided by the three cathode ray tubes on a viewing screen (a 40-inch viewing screen, for example) by using optical lenses or mirror such that the three primary color images are superposed on the viewing screen to produce a color image.
Among electron guns used for the cathode ray tube for the projection TV is one disclosed in Japanese Patent Publication No. Sho 58-31696. This electron gun will be explained by reference to FIG. 5. In FIG. 5, a control grid electrode 102 having a cathode 101 insulatingly secured thereto, an auxiliary focus electrode 103, an accelerating electrode 104, a first anode 110 and a second anode 111 are insulatingly supported on common insulating rods 112, and are housed within a bulb neck portion 107.
The second anode 111 is provided with a small-diameter support end portion 113 at its first anode 110 side end, and supported on the insulating rods 112 adjacent thereto. A tongue-shaped conductive piece 114 attached to a front end portion of the second anode 111 is supplied with a high voltage via a conductive film 115 coated on an inner surface of the bulb neck portion 107, and supplies the high voltage to the second anode 111. The first anode 110 is provided with a neck portion 116 which passes through the small-diameter support end portion 113 of the second anode 111 without contacting the small-diameter support end portion 113, and a large-diameter front end portion 117 which is positioned within a large-diameter cylinder portion of the second anode 111 and has an outside diameter larger than an inside diameter of the small-diameter support end portion 113. The large-diameter front end portion 117 and the large-diameter cylinder portion of the second anode 111 form a main lens therebetween.
With the above structure of the electrodes, both roundness of the second anode 111 and concentricity between the first anode 110 and the second anode 111 can be improved, and further the inside diameter of the second anode 111 can be made close to that of the bulb neck portion 107. Consequently, the large-diameter main lens with spherical aberration reduced can be obtained without increasing the diameter of the bulb neck portion and the diameters of the beam spots can be minimized.
In FIG. 5, reference numeral 118 denotes a faceplate, and reference numeral 119 denotes a phosphor screen formed on an inner surface of the faceplate 118.
Generally, the projection TV receiver requiring three cathode ray tubes and an optical projection system have a disadvantage of greater outside dimensions, and therefore there has been a great demand for reduction of the size of cathode ray tube, especially for the shorter overall length of the cathode ray tubes without deterioration in their focus characteristics.
However, consideration has never been given to shortening of the overall length of the cathode ray tubes in the electron gun for a cathode ray tube described in the Japanese Patent Publication No. Sho 58-31696.
There has been a problem in that generally deterioration in focusing characteristics has been inevitable if the axial length of an electron gun is shortened for shortening of the overall length of the cathode ray tube.
It is an object of the present invention to provide a cathode ray tube capable of shortening the overall length of a cathode ray tube without deteriorating its focusing characteristics by optimizing its electron gun.
A color cathode ray tube in accordance with the present invention achieves the above-mentioned object by a representative structure.
In accordance with an embodiment of the present invention, there is provided a cathode ray tube comprising: a vacuum envelope comprising a panel portion, a neck portion, a funnel portion having a large-diameter end thereof connected to the panel portion and a small-diameter end thereof connected to the neck portion, and a stem having conductive pins embedded therein and closing off an open end of the neck portion on a side thereof opposite from the funnel portion; a phosphor screen formed on an inner surface of the panel portion; an electron gun housed within the neck portion; and a deflection yoke mounted externally of the vacuum envelope in a vicinity of a transition region between the neck portion and the funnel portion; characterized in that the electron gun comprises a cathode, a beam control electrode, an accelerating electrode, a first electrode, a focus electrode and an anode arranged in the order named with specified spacings therebetween, the anode is formed of a large-diameter cylinder portion disposed on a phosphor screen side thereof and a small-diameter cylinder portion disposed on a cathode side thereof, the large-diameter cylinder portion and the small-diameter cylinder portion being connected together in a direction of an axis of the cathode ray tube, the focus electrode is formed of a large-diameter cylinder portion disposed on a phosphor screen side thereof and having a diameter larger than a diameter of the small-diameter cylinder portion of the anode, and a small-diameter cylinder portion disposed on a cathode side thereof and having a diameter smaller than a diameter of the small-diameter cylinder portion of the anode, the large-diameter cylinder portion and the small-diameter cylinder portion of the focus electrode being connected together in the direction of the axis of the cathode ray tube, the large-diameter cylinder portion of the focus electrode being disposed concentrically with and within the large-diameter cylinder portion of the anode, the anode and the first electrode are electrically connected together within the cathode ray tube, and the following inequalities are satisfied: xe2x88x92LP+184.9xe2x89xa7LG4xe2x89xa70.0004 LP3xe2x88x920.1571 LP2+21.006 LPxe2x88x92922.41, LPxe2x89xa7131.7, where LG4 (mm) is an axial length of the focus electrode, and LP (mm) is a distance from a phosphor screen side end of the focus electrode to a center of the phosphor screen.